Yep.Theoretically, 'rated' power' is the power the motor/controller can handle continuously that will cause it to reach thermal equilibrium at a temp lower than will incur damage. Thermal equilibrium occurs where: (heat from power lost due to device efficiency) = (heat dissipated).

This is like water = heat, power is pouring water into a bucket, and heat dissipation is a big leak in the bucket. Up until some point, you can keep pouring water in faster and faster and the bucket will never fill up. At some point, the 'rate in' exceeds the 'rate out' and either slowly or rapidly the bucket will fill up, spill over, and your motor will cook. Similalry, if you back off on the pouring, it takes a bit for the water level to go down (heat to dissipate).

So - a 600W motor should run all day at 600W, but realistically can take thousands of watts for a short burst - but don't 'burst' again repeatedly before the heat bleeds off.

Sadly, motor manufacturers are notorious for bogus ratings. See the ebikes.ca motor and (new) trip simulators to see heat estimates made based on real motor dyno tests.

thanks for the detailed explanation.theoretically, are there other aspects that should be taken into account than thermal equilibrium alone?say heat dissipation is great but a bearing wears off after a few days of continuous operation.is the rated power the electrical input or mechanical output?

joe81 wrote:is the rated power the electrical input or mechanical output?

Good point. It's the mechanical output (rpm x torque). But this brings up the question of which rpm or load should be used, so there is always some grey area over exactly what the 'rated power' really means.

joe81 wrote:theoretically, are there other aspects that should be taken into account than thermal equilibrium alone?say heat dissipation is great but a bearing wears off after a few days of continuous operation.

I believe that if the temperatures are in a safe operating range, then bearing life should be largely unaffected by the cycle life - continuous running is the same as stop and go. There are probably engineering nuances of failure modes that could be brought to bear, but with virtually no moving parts, no serviceable lubrication to (fail to) refresh, and what appears to be only a rough empirical estimate of power capacity by manufacturers, I don't believe those subtleties get much expression in ebike motors...

Some products are over-rated, on order to impress new customers who don't know what they are looking at. Some products are under-Rated, in order to provide a "legal" motor in a place with low power laws, but the device is actually more powerful.

Then, there are the testing parameters. Heavy rider or light? Windy or no wind? Long and steep hills or...short and mild?

This is one of the top reasons forums are important, we can share accurate data with the testing conditions spelled out.

ok. bearings are not such a good example.. but thermal equilibrium could be at such a high temperature that lubrication suffers long term even when temperature is not rising at a certain power level anymore.

as for test conditions i would guess there is an engineering standard like for automotive testing. say ambient temperature 25°C and defined wind speed. ...the later varying with wheel size... yeah maybe too difficult...

now my Chinese kit rated 1800W is ready. the controller puts out a rated current of 22A and the rated voltage is 72V. considering an efficiency of say around 0.85 shouldn't the controller have an electrical output of more like around 30A to achieve a mechanical power of 1.8kW? is my thinking flawed?

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